They laid out a range of flower-shaped feeders holding equal concentrations of sugar solution that varied in temperature. The bees gravitated toward the feeders offering the warmest nectar.

The researchers then placed food solutions in pink or purple feeders.

The bees quickly learned to distinguish between colors and concentrated their foraging on the warmer nectar source.

The results suggest that the bees learned to use color as a signal of temperature and utilized this information in choosing which flowers to visit.

University of Arizona biologist Daniel Papaj says the study is intriguing. But it remains to be seen whether bees in nature commonly use sensory cues to identify warmer flowers, he notes.

If bees do this, Papaj said, "one might expect such behavior to have demographic consequences.

"For instance, flowers in sunnier microhabitats would be more likely to be pollinated."

Bag of Tricks

The fact that pollinating insects may be choosy about temperature suggests a new explanation for a number of features that help plants keep their flowers warmer than the surrounding environment.

A small number of plant species generate heat through metabolic processes, similarly to animals.

Many others use more passive means to gather and retain warmth. The shape of many flowersand the ability to track the sun's movementshelps make them efficient collectors of solar energy.

Some species even have cone-shaped cells in their petals that focus sunlight, increasing the temperature of the flower.

"Many of these features had been previously thought to only enhance the color of the flower, or else warm the flower itself up to help its seed develop faster," said study co-author Heather Whitney of the University of Cambridge in England.

"Now we know that warming structures could be part of the bag of tricks that flowers have evolved to attract pollinators."